FIELD OF THE INVENTION
The invention relates to a steam generator having a combustion chamber for generating a working medium by burning a fuel.
In a fossil-fired steam power plant, the energy content of the fuel is converted into a technologically utilizable form of energy. In the process, a hot gas is generated as a working medium by burning the fuel and the heat content of the hot gas is utilized in order to completely or partly evaporate and/or superheat water in an evaporator. The resulting steam is directed to a steam turbine, where it expands and in the process transmits energy to a generator system.
In such a case, fossil energy carriers in the form of heating oil, natural gas or even coal are used in particular as the fuel. A common feature of those fuels is that they release carbon dioxide (CO.sub.2) during their combustion. The carbon that was converted as a result was combined in the fuel for a considerable length of time before the combustion and was thus extracted from the carbon-dioxide balance of the earth's atmosphere. The combustion of those fuels therefore contributes to an increase in the current global CO.sub.2 level. It is also of particular importance, in particular in view of the greenhouse effect caused thereby, i.e. continuous warming of the earth's atmosphere, to use regenerable energy carriers or energy carriers having a comparatively short carbon-dioxide cycle, such as biomass for example, during the generation of steam. Although carbon dioxide (CO.sub.2) is likewise emitted to the atmosphere during the combustion of biomass, such as straw for example, the carbon forming the basis of that carbon dioxide in general was only incorporated into the biomass a short time before, so that the combustion of the biomass results in no significant increase in the average carbon-dioxide level.
Therefore, straw which is used as a biomass in particular has proven to be an especially attractive fuel for steam generation. However, it proves to be problematic in the construction of a steam generator for straw as a fuel that intensified corrosion of the metallic heating surfaces of the steam generator starts at steam temperatures above about 470.degree. C. due to materials contained in the straw, such as chlorine and potassium, for example. However, in order to be able to operate a steam generator at a sufficiently high efficiency for economical operation, steam temperatures of more than 500.degree. C. are required.
Published European Patent Application 0 206 340 A2, corresponding to U.S. Pat. No. 4,681,065, and U.S. Pat. No. 3,884,193, respectively disclose a steam generator having a first combustion chamber for generating a working medium by burning a first fuel and having a second combustion chamber for burning biomass as a fuel. In the case of such steam generators, the second combustion chamber in each case is connected downstream of the first combustion chamber in the flow direction of the working medium.